The present invention pertains to high efficiency switching power supplies, particularly to flyback designs.
Switching power supplies are widely used in electronic devices. They convert unregulated power from batteries or other sources to a regulated voltage required to operate the device. Especially with battery operated hand held and portable devices, high efficiency is required to maximize battery life. When the input voltage may vary either below or above the desired output voltage, or isolation is desired between input and output, a flyback topology is commonly used.
The flyback topology uses a transformer and a switch fed from a direct current source. When the switch is closed, current flows in the primary winding of the transformer, building up magnetic flux in the core. When the switch is opened, the polarity of the voltage reverses, causing current to flow in one or more secondary windings of the transformer. This current induced in the secondary windings is rectified and filtered to produce DC output voltages. Output voltage regulation is obtained by using a feedback circuit from a selected output voltage to control the pulse width or the frequency of the switching action.
MOSFETs are commonly used as switches in switching power supplies. The on resistance of a MOSFET switch used in a flyback power supply is dependent its the gate to source voltage. At lower input voltages, such as in battery operation from 1 or 2 cells, the gate drive voltage is lower, and the on resistance of the MOSFET higher. This lower gate drive voltage results in higher on resistance, and introduces inefficiency in the operation of the supply, especially at low input voltages. Also, until recently, MOSFETs required a gate to source voltage of approximately 3 volts to achieve low on resistance.
What is needed is a flyback switching power supply that provides a high gate drive voltage, preferably higher than the input voltage, without a separate power supply to generate this higher voltage.